Completion fluids are composed of a variety of brines of different salinity characterized by a density ranging from 8.5 pounds per gallon (ppg) for seawater to 19.2 ppg for heavy brines containing zinc bromide and calcium bromide. Currently, high viscosity grade hydroxyethylcelluloses (HECs), such as Natrosole HEC 250 HHR-P, Natrosol® HEC HI-VIS and Natrosol® HEC 210 HHW (available from Hercules Incorporated) are used as thickeners for brines having density from 9-13 ppg. These Natrosol® HECs are presently manufactured using purified cotton linters and are characterized by a 1% aqueous solution having a Brookfield viscosity exceeding 5,000 cPs. For lower density, non-calcium based brines, xanthan gum is the polymer of choice for its high carrying capacity and gel strength
Currently, there is no efficient viscosifier for heavy brines with a density ranging from 14 ppg (CaBr2) to 19.2 ppg (ZnBr2/CaBr2). These brines have a very low level of free water, and therefore, do not promote optimum hydration of standard HECs. As the salt content in the brine solution is increased, the hydration rate of the HEC dramatically decreases, and it takes longer time to build the desired viscosity. Furthermore, these brines are characterized by a very low pH (pH<1 for ZnBr2/CaBr2).
Obviously, there is a need to develop an HEC or an analogous polymer that has very high solution viscosity and dissolves rapidly in high density brines. While technically it is possible to increase the viscosity of the HECs by reducing their average oxyethylene molar substitution (MS), the manufacture of such low MS HECs is difficult.